This invention relates to an optical system having a window therein, and in particular to such an optical system used in an aircraft or missile wherein the window is a conformal window.
An optical sensor receives radiated energy from a scene and converts it to an electrical signal. The electrical signal is provided to a display or further processed for pattern recognition or the like. Optical sensors are available in a variety of types and for wavelengths ranging from the ultraviolet, through the visible, and into the infrared. Optical sensors are used in a variety of commercial and military applications. In some applications the optical sensors are fixed in orientation, and in others the optical sensor is movable such as by a pivoting motion to allow sensing over a wide angular range.
The optical sensors generally employ a photosensitive material that faces the scene and produces an electrical output responsive to the incident energy. The photosensitive material and remainder of the sensor structure are rather fragile, and are easily damaged by dirt, erosion, chemicals, or high air velocity. In service, the sensor is placed behind a window through which it views the scene and which protects the sensor from such external effects. The window must be transparent to the radiation of the operating wavelength of the sensor and resist attack from the external forces. The window must also permit the sensor to view the scene over the specified field of regard.
The window would ideally introduce no wavefront aberration at the center of the field of view, other than possibly spherical aberration, particularly if the sensor is an imaging sensor. The thicker and more highly curved is the window, the more likely is the introduction of significant wavefront aberration. A wide variety of sensor windows have been used in various aircraft applications. In many cases such as low-speed commercial helicopters, flat windows are acceptable. Windows that are shaped as segments of spheres are used in aircraft and missile applications, but for these windows the wavefront aberration tends to be high if the gimbal location is not at the spherical center of the window. In all of these window types, if the window must be wide or must project a substantial distance into an airflow to permit a large field of regard, the aerodynamic drag introduced by the window is large.
For applications involving aircraft and missiles operating at high speeds, the window should be relatively aerodynamic such that the presence of the window extending into the airstream does not introduce unacceptably high and/or asymmetric aerodynamic drag to the vehicle. A conformal window is therefore beneficial to reducing drag and increasing the range of the aircraft. Some existing conformal windows introduce large wavefront aberrations into the sensor beam, particularly for high azimuthal pointing angles of the sensor.
An important consideration in achieving acceptable cost of the optical system is that the conformal window must be easily tested for its accuracy of shape, and must also be readily aligned upon mounting in the flight vehicle. The more complex the shape of the conformal window, the greater the challenge in testing and alignment.
There is a need for an improved window to be used in conformal window applications in high-speed missiles and aircraft. The present invention fulfills this need, and further provides related advantages.